A signal is simply the transmission of data from one place to another place. In our day to day life we deal with various signals constantly like signals from music, power lines, telephones, and cellular devices.
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Now about the topic of our consideration i.e. analog and digital signals are two kinds of processes which are used for the transmission of information signals from source to destination. Usually the information to be transmitted from one place to another is either audio or video. This information signal is then transformed into those signals which can be transmitted via different channels i.e. medium of communication.
For the analog format, the data is transformed into electrical pulses with varying amplitude while for the digital format; the data is transformed into binary format representing two amplitudes.
Devices come with built-in ‘translation’ facilities. We have various such equipment like analog or digital phones, fax machines, modems, clocks, watches etc. A microphone and speaker are good examples for analog devices.
Analog technology is older one and has been used for decades. It is cheap too but the problem with analog signals is that there is a limitation on the size of the data that can be transmitted at any given point of time.
With the advent of digital technology many improvements and new techniques have been introduced. Now a days almost every appliance or equipment is based on digital technology. In this, the transmitter translate the data into binary form and the receiver re assemble and produces the original information signal.
Digital signal as compared to analog gives us more options as it can be easily computed and manipulated by software. Also now day’s digital signals are frequently used in telecommunication.
Instead of having various advantages over analog signal, digital signal lag behind in quality. Since in digital devices there is translating and reassembling of data because of this reason its quality is not that good. But with new technologies we can possibly remove errors and noise in the digital signal. But digital is quite expensive as compared to analog and working is going on globally to reduce its price.
II. ANALOG SIGNALS
Analog signals are continuous electrical signals that vary with time as shown in figure. In other words an Analog signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e. analogous to another time varying signal. It produces small fluctuations in the signal which are very useful and this lacks in digital signals.
Not all analog signals vary as smoothly as the waveform shown in Figure. There are small variations in the analog signal.
An analog signal uses some property of the medium to convey the signal’s information. For example, an aneroid barometer to convey pressure information uses rotary position as the signal. Electrically, the property most commonly used is voltage followed closely by frequency, current, and charge.
Any information may be conveyed by an analog signal; often such a signal is a measured response to changes in physical phenomena, such as sound, light, temperature, position, or pressure, and is achieved using a transducer.
The resolution of analog signals is infinite. In real world, an analog signal is subject to noise and a finite slew rate. Therefore, both analog and digital systems are subject to limitations in resolution and bandwidth. As analog systems become more complex, effects such as non-linearity and noise ultimately degrade analog resolution to such an extent that the performance of digital systems may surpass it.
Telephone voice signal is analog. The variations in the analog signal are due to changing intensity in voice. At the receiving end, the signal is reproduced in the same proportion.
A. Analog signal transmission
Although now a day’s, every equipment uses digital technology but still analog technology is used. The laboratories and some manufacturing companies still uses the old technology i.e. analog electric signals. And a fundamental understanding of how analog signal transmission works must first begin with a discussion of electrical basics.
Before we go deep into analog signal transmission firstly we have to understand the relationship which makes analog signal transmission possible. It is the fundamental relationship between voltage, current, and electrical resistance that allow either a continuously varying current or voltage to represent a continuous process variable.
Electric current is flow of electrons and voltage is the work done in moving a unit of charge (1 coulomb) from one point to another. The unit of voltage is often called the potential difference, or the volt (V). The SI unit of electric current is Ampere denoted by ‘A’ and is defined as one coulomb per second (c/s).
A source of voltage, V, will cause a current, I, to flow through a resistor of resistance, R. Ohm’s law, which was formulated by the German physicist Georg Simon Ohm defines the relation:
While most single-channel analog signal transmissions use direct current (dc) variations in current or voltage to represent a data value, frequency variations of an alternating current (ac) also can be used to communicate information.In the early 19th century, Jean Baptiste Joseph Fourier, a French mathematician and physicist, discovered that ac signals could be defined in terms of sine waves. A sine wave is described by three quantities: amplitude, period, and frequency. The amplitude is the peak value of the wave in either the positive or negative direction, the period is the time it takes to complete one cycle of the wave, and the frequency is the number of complete cycles per unit of time (the reciprocal of the period).
B. Advantages of analog signals
* The main advantage of analog signals is their fine definition which has the potential for an infinite amount of signal resolution. In comparison to digital signals, analog signals have higher density.
* Analog signals are easier and simpler to process as compared to complex digital signals. An analog signal can be processed directly by analog components, though some processes aren’t available except in digital form.
* Analog signals are relatively easy to create and carry from place to place.
C. Disadvantages of analog signals
The major disadvantage of analog signaling is that any system has noise, any random unwanted variation. As the signal is copied and re-copied, or transmitted over long distances, these random variations interfere with our signal and distort it. Electrically, these losses due to interference of noise can be diminished by shielding, good connections, and several cable types such as coaxial or twisted pair.
In the figure the blue lines represents the voltage capacity of signal to travel clearly and for this it must lie between minimum value of X and maximum value of Y.
Here the noise had distorted our signal. Noise is unwanted variations in the signal which deteriorates the quality of signal. The signal had crossed the limits of X and Y and lost its authentication.Noise is sometimes called “distortion” or “clipping.”
We can say that Noise creates loss in authentic information and produces distortion. If the pattern of our original signal is slightly altered by unwanted noise ordistortion, the output will not be identical to the input.
This is impossible to recover, because when we amplify the signal to recover attenuated parts of the signal amplifies the noise (distortion/interference) as well. Although the resolution of analog signals is higher than digital signals but the difference can be overshadowed by the noise in the signal.
III. DIGITAL SIGNALS
Digital signals are the one which are non-continuous, change in individual steps. They consist of pulses or digits with discrete levels or values. The value of each pulse is constant, but there is an abrupt change from one digit to the next. Digital signals have two amplitude levels called nodes. The value of which are specified as one of two possibilities such as 1 or 0, HIGH or LOW, TRUE or FALSE and so on. In reality, the values are anywhere within specific ranges and we define values within a given range.
Discrete time signals-
An analog signal is a datum that changes over timeââ‚¬”say, the temperature at a given location; the depth of a certain point in a pond; or the amplitude of the voltage at some node in a circuitââ‚¬”that can be represented as a mathematical function, with time as the free variable (abscissa) and the signal itself as the dependent variable (ordinate). A discrete-time signal is a sampled version of an analog signal: the value of the datum is noted at fixed intervals (for example, every microsecond) rather than continuously.
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If individual time values of the discrete-time signal, instead of being measured precisely (which would require an infinite number of digits), are approximated to a certain precisionââ‚¬”which, therefore, only requires a specific number of digitsââ‚¬”then the resultant data stream is termed a digital signal. The process of approximating the precise value within a fixed number of digits, or bits, is called quantization.
In conceptual summary, a digital signal is a quantized discrete-time signal; a discrete-time signal is a sampled analog signal.
In the Digital Revolution, the usage of digital signals has increased significantly. Many modern media devices, especially the ones that connect with computers use digital signals to represent signals that were traditionally represented as continuous-time signals; cell phones, music and video players, personal video recorders, and digital cameras are examples.
In most applications, digital signals are represented as binary numbers, so their precision of quantization is measured in bits. Suppose, for example, that we wish to measure a signal to two significant decimal digits. Since seven bits, or binary digits, can record 128 discrete values (viz., from 0 to 127), those seven bits are more than sufficient to express a range of one hundred values.
A. Transmission of digital signals
There are two ways to transmit digital data between one or several devices or communication participants, either parallel or serial transmission.
* Bit-parallel transmission:
In bit parallel transmission all the bits of information signal are transmitted at once at the same time. Bit-parallel via an appropriate number of signal lines.
The cost of installation is high and is viable only for short distances. The transmission of one byte of data requires a minimum of nine lines, 8 bits and a reference potential.
Therefore, this technique is presently almost only used for device busses. This application over short distances requires high transmission rates while doing without conversion methods that need a large number of components.
* Bit-serial transmission
Serial transmission is a good for long distances. In this case, only one signal line transmits the bits one after the other. As a result, the transmission of information takes more time, which is nevertheless acceptable because, on the other hand, the installation effort and the costs are considerably reduced.
Since all the information is mostly generated and processed in bit-parallel mode, the transmitter must convert the data from parallel to serial, and the receiver must reconvert it from serial to parallel. This function is performed by specially operated shift registers which are already integrated in communication modules available on the market.
B. Advantages of digital signal over analog signal
Digital signals consist of patterns of bits of information. These patterns can be generated in many ways, each producing a specific code.
Modem digital computers store and process all kinds of information as binary patterns. All the pictures, text, sound and video stored in this computer are held and manipulated as patterns of binary values.
Here the signal acquire two basic forms i.e. ON (high or 1) and OFF (low or 0). If we compare with analog these digital signals are more uniform.
Here, we see the main advantage of digital over analog. Since the signal is very uniform, noise has not severely altered its shape or amplitude. The digital signal shows a far less change to the actual waveform than the previous analog signal. They are both shown below for a close comparison.
Thus we can say that main advantage of digital signals over analog signals is that the precise signal level of the digital signal is not vital. This means that digital signals are fairly immune to the imperfections of real electronic systems which tend to spoil analog signals as shown above. As a result, digital CD’s are much more robust than analog LP’s.
Codes are often used in the transmission of information. These codes can be used either as a means of keeping the information secret or as a means of breaking the information into pieces that are manageable by the technology used to transmit the code, e.g. The letters and numbers to be sent by a Morse code are coded into dots and dashes.
Thus we can say that though digital technology is expensive as compared to that of analog but because of various advantages and more options it has made analog technology redundant.
C. Disadvantage of digital signals
The one main drawback of digital communication is that they require greater bandwidth as compared to analog communication for the transmission of same signal.
In conclusion, the strength of using a digital system over analog is clear. Digital signals are easier to transmit and offer less room for errors to occur i.e. vary less distortion in our original signal. This leads to accurate data transmission that in turn leads to faster transmission rates and better productivity.
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